Which of these numbers is prime? ${4,\ 15,\ 25,\ 55,\ 89}$
A prime number has exactly two factors: 1 and itself. The factors of 4 are 1, 2, and 4. The factors of 15 are 1, 3, 5, and 15. The factors of 25 are 1, 5, and 25. The factors of 55 are 1, 5, 11, and 55. The factors of 89 are 1 and 89. Thus, 89 is a prime number.